In the manufacture of optical fiber, a glass preform is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and a glass fiber is drawn freely from the molten end of the preform by a capstan located at the base of a draw tower. The present invention is concerned with the making of such a glass preform, which is a solid cylindrical rod having a refractive-index profile (i.e., the variation of the index of refraction as a function of distance from the center of the rod) that is suitable for guiding light. It is noted that the refractive-index profiles of the preform and the drawn fiber are substantially identical, even though the preform has a diameter that is thousands of times larger than that of the drawn fiber. The amount of fiber that can be drawn from a glass preform is directly proportional to the size of the preform. And there are significant cost savings attributable to the use larger preforms.
There are a number of competing processes for fabricating glass preforms, one of them being known as modified chemical vapor deposition (MCVD) in which the index of refraction of the preform is developed by depositing glassy particles (soot) on the inside walls of a glass tube (sometimes called a substrate tube). The soot comprises silica that is generally doped with germanium to increase its index of refraction. Other popular processes for making preforms include Outside Vapor Deposition (OVD) and Vapor Axial Deposition (VAD) in which soot is deposited on the outside surface of a soot boule that is sintered in a subsequent step.
There is perceived limitation on MCVD preform size that is due to the relatively low glass deposition rates and the relatively small amount of deposited glass possible in an inside deposition process. During the MCVD process, soot is simultaneously deposited and sintered into glass on the inside surface of the substrate tube. An MCVD heat source is typically a reciprocating oxygen-hydrogen torch that heats the outside surface of the substrate tube. Each pass of the torch adds a thin layer of glass onto the inside surface of the tube. Multiple thin layers of cladding material are deposited first, and then multiple thin layers of core material are deposited. In the simplest singlemode designs (i.e., non-dispersion-shifted, matched-clad or depressed-clad fiber), the cladding is usually at one level, which may be achieved by fluorine and/or phosphorus doping and the core is at another (usually just germanium doping). In more complicated singlemode designs (i.e., dispersion shifted fiber), the refractive index can vary across both the cladding and the core. Differences are created by varying the concentration of the dopant material (e.g., germanium) that is used. Limitation on the heat transfer rate across the substrate tube wall and on the maximum amount of reactants that can be delivered down the tube and efficiently reacted in the hot zone restrict the deposition rates. Typical MCVD deposition rates are between 0.2 and 2.0 grams/minute--depending on the composition of the glass. The diameter of the deposited core is designated (d) and the diameter of the deposited cladding is designated (D). Ideally, only core material needs to be deposited on the inside wall of the substrate tube to create the desired refractive index profile. In this situation, the deposited cladding/core (D/d) ratio is 1.00. However, this places too severe a requirement on the purity of the substrate tube. Presently, preforms made by MCVD yield up to 250 kilometers (km) of fiber per meter of preform length. By way of contrast, the deposition rate for the OVD and VAD processes range between 5 and 50 grams/minute, and preforms made by OVD and VAD yield more than 400 km of fiber per meter of preform length.
Nevertheless, MCVD has certain advantages relative to the above-mentioned outside deposition processes. Since soot deposition and sintering occur simultaneously in MCVD, dopants can be incorporated into the glass and fixed in place on a layer by layer basis. Moreover, the range of dopants that can be used in MCVD is larger than that of the outside processes. In addition to those dopants common to all processes, such as germanium, which raise the index of refraction, fluorine doping can be used in MCVD to significantly lower the index of refraction. Fluorine doping is difficult to manage in outside processes because the incorporation in soot is basically a diffusion process. The versatility of MCVD with respect to dopant choices allows the straightforward construction of complex index profile shapes. Another advantage of MCVD, which partially offsets the deposition rate disadvantage, is the use of the substrate tube and subsequent overclad tube to provide the bulk of the glass in the overall fiber manufacturing process.
In the MCVD process, dimensional instabilities and the formation of bubbles are created in a substrate tube that is exposed to extremely high temperatures for long periods of time, as would be the case in the manufacture of large MCVD preforms because large amounts of core material need to be deposited within the substrate tube. If one could merely increase the deposition rate of core material, as is done in MCVD preforms for multimode fiber where the deposition rate exceeds one gram per minute, then this concern would disappear. However, unlike multimode fiber which uses high levels of GeO.sub.2 doping material, singlemode fiber uses relatively low levels of GeO.sub.2. This difference is important because higher levels of dopant tend to decrease the softening point temperature (viscosity) of the deposited materials so that they form a smooth layer on the inside wall of the substrate tube at lower temperature. Accordingly, sintering can be accomplished at lower temperatures. Additionally, multimode fiber preforms further include P.sub.2 O.sub.5 as a doping material, which further decreases the softening point temperature of the deposited materials (see, for example, U.S. Pat. No. 4,339,173).
Accordingly, what is sought is an MCVD preform for singlemode fiber that yields about 400 km, or more, of fiber per meter of preform length, and a commercially attractive process for making same.